More than 360 000 people have been killed across war-ravaged Syria in seven years, a monitoring group said on Thursday, in a new toll for the brutal conflict.

It came amid rising international concern that a looming Syrian government assault against rebels in the northwest province of Idlib would be a "bloodbath".

The Syrian Observatory for Human Rights said it had recorded the deaths of 364 792 people, nearly a third of them civilians, since protests erupted in March 2011 against President Bashar al-Assad.

The toll represents an increase of about 13 000 people in the past six months, according to the Britain-based monitor, which uses a vast network of sources including fighters, officials and medical staff.

The war has killed 110 687 civilians, including more than 20 000 children and nearly 13 000 women.

More than 124 000 pro-government fighters have died, around half of them regime troops and the rest an assortment of Syrian and foreign militiamen loyal to Assad.

Among them are 1665 from Lebanon's Hezbollah movement.

The Observatory recorded the deaths of 64 000 hardline Islamists and jihadists, including from the Islamic State group and former al-Qaeda affiliate factions.

Another 64 800 fighters from other forces, including non-jihadist rebels, soldiers who defected and Kurdish factions, were also killed since 2011.

The Observatory said it had confirmed the deaths of another 250 people but could not specify their identities.

With help from his Russian and Iranian allies, Assad has recaptured nearly two-thirds of Syrian territory.

The lion's share of the rest is the Kurdish-controlled northeast.

The largest chunk of rebel-held territory left comprises the province of Idlib and surrounding areas, where an estimated three million people live.

Assad's troops have been amassing around the area for weeks ahead of a threatened assault.

The United Nations, world powers, and aid groups alike have warned a full-fledged offensive on Idlib could create a humanitarian calamity.

UN Secretary General Antonio Guterres this week urged the regime to pull back and for all sides to find a peaceful solution, saying Idlib "must not be transformed into a bloodbath".